Calculation of wire cross-section. Which cable is better to use for wiring in an apartment: brands, sections, choice Calculation of the cross-section of the input cable

Electricity has become such a part of our lives that when there is a sudden power outage, our lives seem to freeze, and we impatiently wait for it to be restored. We are surrounded by a huge number of different electrical appliances that connect to the home network either directly through sockets or through

extension cords or carriers.

Sometimes it is necessary to either move it to another outbuilding, replace the wire, or make a homemade extension cord. Or you need to calculate what is the maximum number of devices that can be simultaneously connected to one tee so that the wire does not heat up in it and a fire does not occur due to In such cases, you should first do this to be sure of the safety of the installed electrical wiring.

Which wire to choose?

It is no secret that copper has less resistance compared to aluminum, and therefore, if you compare copper and those with the same wire cross-section, then in the first case the permissible load will be slightly greater. Copper wire is stronger, softer and does not break at bend points. In addition, copper is less susceptible to oxidation and corrosion. The only advantage of aluminum wire is its cost, which is three or four times less than copper wire.

Calculation of wire cross-section by power

Any electrical wiring must be suitable for the load connected to it. The cross-section of the wire is calculated based on the maximum permissible heating of the current-carrying conductor. The amount of heating depends on the power of the connected electrical appliances. Thus, by calculating the maximum possible total power of the devices in the room, you can determine what the wire cross-section should be. In practice, it is convenient to use an online calculator or special tables that contain information about the permissible current load on the cable.

Wire cross-section, sq.mm	Copper wire (cable, core)
	Circuit voltage, 220 V		Circuit voltage, 380 V
	power, kWt	current strength, A	power, kWt	current strength, A

Wire cross-section, sq.mm	Aluminum wire (cable, core)
	Circuit voltage, 220 V		Circuit voltage, 380 V
	power. kW	current strength. A	power. kW	current strength. A

How to check the wire cross-section?

Since wires most often have a round cross-section, the cut area is calculated using the formula:

S = π x d²/4 or S = 0.8 x d², where
S - cross-sectional area of ​​the core in mm2;
π - 3.14;
d—core diameter in mm.

Let's say, for example, that the diameter of the wire is 1.3 mm, then S = 0.8. 1.3² = 0.8. 1.3 x 1.3 = 1.352 mm2

If the wire consists of several cores, then the cross-section of one core is calculated and multiplied by their total number in the bundle. The diameter is usually measured with a caliper, but if you don't have one, a regular ruler will do. In this case, approximately 10-15 turns are tightly wound around a pencil, the length of the winding is measured with a ruler and the resulting value is divided by the number of turns.

When performing any electrical installation work, you must remember that electricity does not tolerate careless handling and does not forgive mistakes. Electrical safety and reliability - this is what you should always strive for when doing any work with electrical wiring in an apartment, country house or house.

It is important to select the wire cross-section for those who independently conduct electricity into the house.

The correctness of the cross-section determines the uninterrupted supply of current, the absence of risks of failures, the stability of the devices, as well as the safety of the residents of the house, which is important for human life.

If you take the wrong wire, that is, choose the wrong cross-section, the following consequences may occur:

• the cable will overheat;
• high temperature will cause the insulation to melt;
• there is a risk of occurrence;
• possible fire;
• Devices powered by the network may burn out during operation.

How to choose wires?

Long-term permissible current load is the main thing you should pay attention to when choosing a wire in a store.

Each type of wire must be supplied for sale with a passport, where all this data is indicated.

What is the continuous load capacity? This is the maximum total power of devices that consume current.

If the limit is exceeded, operation of the wiring is unacceptable.

So, it is logical to assume that to calculate the required cross-section we take into account the total power of appliances, devices and other items that consume energy (even the most common phone chargers need to be taken into account).

If we carry out, it is better to install the wiring with a reserve of maximum throughput, since the repairs are being done, probably, not for one year, but over time, the devices are made more powerful, and you will probably want to buy something additional.

Speaking about the effectiveness of copper or aluminum in the production of cables for electrical wiring, it is worth confidently saying that aluminum has a greater set of advantages compared to copper. In its favor we can note:

• resistant to mechanical damage;
• does not break when bent;
• durable;
• flexible;
• no oxidation;
• If you compare copper and aluminum in operation, then two different wires with the same cross-section will be able to transmit different amounts of energy. Of course, copper wins the battle for the right to a more reliable component.

If the most powerful devices are planned to be connected to different sockets, then the wire cross-section can be 2.5 mm at the load that we showed in the example.

If, with the same indicator, high-power devices are connected to one outlet (or even to one room), then 4-6 mm is the ideal solution.

However, for a room where too strong appliances will not operate, a cross-section of 1.5 mm is sufficient for the entire room.

You also have to figure out... The diagram will help with this:

As a rule, the main thing in an apartment with two rooms that is powered by electricity is:

• boiler. The most powerful device, but not everyone has it. If you have a gas water heater installed, but in the future you plan to switch to a boiler, it is better to immediately take into account that it consumes about 2000 W;
• iron. Although we turn it on infrequently, this device consumes as much as 1700 W, which will significantly affect the amount of energy flow when it is turned on;
• Electric kettle. Consumes 1200 W. A kitchen attribute in almost every apartment;
• washing machine. Perhaps one of the leaders in energy intake. Consumes 2500 W;
• microwave oven - power varies, but averages 700 W;
• vacuum cleaner. Approximately 650 W;
• computer. 500 W;
• light. 500 W;
• fridge. 300 W;
• modern TV. 140 W.

Important: there are energy-saving devices, and there are ordinary ones. Radiators that look the same can vary significantly in the level of energy consumed, but on the box or in the passport for the device, this data must be indicated based on how much time the device consumes per hour.

Calculation of wire cross-section for a network phase

For single phase

• sum up the power of all devices that will be used in the apartment;
• we multiply the resulting amount by the simultaneity coefficient (this indicator is calculated based on the average data on the simultaneous activation of a number of devices, and is 0.75);
• divide the resulting number by the network voltage (in our case 220).

Calculation of wire cross-section for a three-phase network 380 W

We calculate in the following sequence:

In general, the formula will look like this:

Wire cross-section table

To find out the permissible load for a specific wire and calculate the wire cross-section, it is enough to compare the obtained data with ready-made tables.

True, a lot depends on what wire is used.

For copper wires:

For aluminum:

Important: if the cable consists of 4 or 5 cores, the result obtained is multiplied by a factor of 0.93.

Here you can learn everything from real professionals in their field.

So, the known power of each electrical appliance in the house, the known number of lighting fixtures and lighting points allow us to calculate the total power consumed. This is not an exact sum, since most values ​​for the powers of various devices are averages. Therefore, you should immediately add 5% of its value to this figure.

Average power readings for common electrical appliances

Consumer	Power, W
TV	300
Printer	500
Computer	500
Hair dryer	1200
Iron	1700
Electric kettle	1200
Toaster	800
Heater	1500
Microwave	1400
Oven	2000
Fridge	600
Washing machine	2500
Electric stove	2000
Lighting	2000
Instantaneous water heater	5000
Boiler	1500
Drill	800
Hammer	1200
Welding machine	2300
Lawnmower	1500
Water pump	1000

And many believe that this is enough to select almost standard copper cable options:

• cross section 0.5 mm2 for wires for lighting spotlights;
• cross section 1.5 mm2 for lighting wires for chandeliers;
• cross-section 2.5 mm2 for all sockets.

At the level of household use of electricity, such a scheme looks quite acceptable. Until the refrigerator and electric kettle decided to turn on in the kitchen at the same time, while you were watching TV there. The same unpleasant surprise overtakes you when you plug in a coffee maker, washing machine and microwave into one outlet.

Thermal calculation using correction factors

For several lines in one cable channel, the tabulated values ​​of the maximum current should be multiplied by the appropriate coefficient:

• 0.68 — for the number of conductors from 2 to 5 pcs.
• 0.63 — for conductors from 7 to 9 pcs.
• 0.6 — for conductors from 10 to 12 pcs.

The coefficient refers specifically to the wires (cores), and not to the number of passing lines. When calculating the number of laid wires, the neutral working wire or grounding wire is not taken into account. According to PUE and GOST 16442-80, they do not affect the heating of wires during the passage of normal currents.

Summarizing the above, it turns out that in order to correctly and accurately select the wire cross-section, you need to know:

1. The sum of all maximum powers of electrical appliances.
2. Network characteristics: number of phases and voltage.
3. Characteristics of cable material.
4. Tabular data and coefficients.

At the same time, power is not the main indicator for an individual cable line or the entire internal power supply system. When selecting a cross-section, be sure to calculate the maximum load current, and then check it with the rated current of the home circuit breaker.

How to calculate a cable by current, voltage and length. , as you know, come in different sections, materials and with different numbers of cores. Which one should you choose so as not to overpay, and at the same time ensure the safe, stable operation of all electrical appliances in the house? To do this, it is necessary to calculate the cable. The cross section is calculated by knowing the power of the devices powered from the network and the current that will flow through the cable. You also need to know a few other wiring parameters.

Basic Rules

When laying electrical networks in residential buildings, garages, and apartments, rubber or PVC insulated cables designed for a voltage of no more than 1 kV are most often used. There are brands that can be used outdoors, indoors, in walls (grooves) and pipes. Usually this is a VVG or AVVG cable with different cross-sectional areas and number of cores.
PVA wires and SHVVP cords are also used to connect electrical appliances.

After calculation, the maximum permissible cross-section value is selected from a number of cable grades.

Basic recommendations for choosing a cross-section are found in the Electrical Installation Rules (PUE). The 6th and 7th editions have been released, which describe in detail how to lay cables and wires, install protection, distribution devices and other important points.

For violation of the rules, administrative fines are provided. But the most important thing is that violation of the rules can lead to failure of electrical appliances, fire of wiring and serious fires. Fire damage is sometimes measured not in monetary terms, but in human casualties.

The importance of choosing the right section

Why is cable sizing so important? To answer, you need to remember your school physics lessons.

Current flows through the wires and heats them. The stronger the power, the greater the heating. Active current power is calculated using the formula:

P=U*I* cos φ=I²*R

R– active resistance.

As you can see, power depends on current and resistance. The greater the resistance, the more heat is generated, that is, the more the wires heat up. Same for current. The larger it is, the more the conductor heats up.

Resistance in turn depends on the material of the conductor, its length and cross-sectional area.

R=ρ*l/S

ρ - resistivity;

l– length of the conductor;

S– cross-sectional area.

It can be seen that the smaller the area, the greater the resistance. And the greater the resistance, the more the conductor heats up.

If you buy a wire and measure its diameter, do not forget that the area is calculated using the formula:

S=π*d²/4

d– diameter.

Don't forget resistivity either. It depends on the material from which the wires are made. The resistivity of aluminum is greater than that of copper. This means that with the same area, aluminum will heat up more strongly. It immediately becomes clear why it is recommended to use aluminum wires with a larger cross-section than copper wires.

In order not to go into a long calculation of the cable cross-section each time, standards for selecting the cross-section of wires in tables have been developed.

Calculation of wire cross-section by power and current

The calculation of the wire cross-section depends on the total power consumed by electrical appliances in the apartment. It can be calculated individually, or use average characteristics.

For the accuracy of calculations, a block diagram is drawn up that shows the devices. You can find out the power of each from the instructions or read on the label. Electric stoves, boilers, and air conditioners have the highest power. The total figure should be in the range of approximately 5-15 kW.

Knowing the power, the rated current is determined using the formula:

I=(P*K)/(U*cos φ)

P– power in watts

U=220 Volt

K=0.75 – simultaneous switching factor;

cos φ=1 for household electrical appliances;

If the network is three-phase, then a different formula is used:

I=P/(U*√3*cos φ)

U=380 Volt

Having calculated the current, you need to use the tables that are presented in the PUE and determine the cross-section of the wire. The tables indicate the permissible continuous current for copper and aluminum wires with various types of insulation. Rounding is always done upward to allow for margin.

You can also refer to tables in which the cross-section is recommended to be determined only by power.

Special calculators have been developed that can be used to determine the cross-section, knowing the power consumption, the phase number of the network and the length of the cable line. You should pay attention to the installation conditions (in a pipe or outdoors).

Influence of wiring length on cable selection

If the cable is very long, then additional restrictions arise on the choice of cross-section, since voltage losses occur over an extended section, which in turn lead to additional heating. To calculate voltage losses, the concept of “load torque” is used. It is defined as the product of power in kilowatts and length in meters. Next, look at the value of losses in the tables. For example, if the power consumption is 2 kW and the cable length is 40 m, then the torque is 80 kW*m. For copper cable with a cross-section of 2.5 mm². this means that the voltage loss is 2-3%.

If the losses exceed 5%, then it is necessary to take a cross section with a margin greater than that recommended for use at a given current.

Calculation tables are provided separately for single-phase and three-phase networks. For three-phase load torque increases, since the load power is distributed over three phases. Consequently, losses are reduced and the effect of length is reduced.

Voltage losses are important for low-voltage devices, in particular gas-discharge lamps. If the supply voltage is 12 V, then with a loss of 3% for a 220 V network, the drop will be little noticeable, and for a low-voltage lamp it will be almost halved. Therefore, it is important to place ballasts as close as possible to such lamps.

Calculation of voltage losses is performed as follows:

∆U = (P∙r0+Q∙x0)∙L/ Un

P— active power, W.

Q— reactive power, W.

r0— active resistance of the line, Ohm/m.

x0— line reactance, Ohm/m.

Un– rated voltage, V. (it is indicated in the characteristics of electrical appliances).

L— line length, m.

Well, if it’s simpler for everyday conditions:

ΔU=I*R

R– cable resistance, calculated using the well-known formula R=ρ*l/S;

I– current strength, found from Ohm’s law;

Let's say we get that I=4000 W/220 IN=18.2 A.

The resistance of one strand of copper wire with a length of 20 m and an area of ​​1.5 mm2. amounted to R=0.23 Ohm. The total resistance of the two wires is 0.46 Ohms.

Then ΔU=18.2*0.46=8.37 V

Percentage

8,37*100/220=3,8%

On long lines against overloads and short circuits they are installed with thermal and electromagnetic releases.

Calculation of wire cross-section is a very important component of high-quality and reliable electrical wiring. After all, these calculations include the power consumption of electrical equipment and the long-term permissible currents that the wire can withstand in normal operating mode. In addition, we all want to have a guarantee and be confident in the electrical and fire safety of electrical wiring, so wire cross section calculation is so important.

Let's see what the wrong choice of wire cross-section can lead to.

In most cases, electricians currently working on the market in this service sector do not bother themselves with performing any calculations at all, but simply overestimate or underestimate the cross-section of the wire. This is usually due to the fact that, after a long time after graduating from educational institutions, they do not remember how to do this, since the acquired knowledge was not consolidated in practice in a timely manner. For the most part, this knowledge is possessed by a certain portion of power engineers and chief engineers, and this is due to the fact that their knowledge is exploited in this direction every day.

If the wire cross-section is less than required

Let's consider an example if the wire cross-section is underestimated, that is, less power consumption is selected.

This case is the most dangerous of all those considered, as it can lead to damage to electrical equipment, fire, electric shock to people, and often death. Why this happens is very simple. Let's say we have an electric water heater with a power of 3 kW, but the wire installed by a specialist can withstand only 1.5 kW. When you turn on the water heater, the wire will become very hot, which will eventually lead to damage to the insulation, and subsequently to its complete destruction, and a short circuit will occur.

If the wire cross-section is larger than required

Now, let's look at an example with an oversized wire cross-section, chosen larger than what is required for the equipment. People even have all sorts of sayings about reserve, they say it’s not superfluous. Within reasonable limits, it is really not superfluous, but it will cost much more than required. For the 3 kW water heater given in the example above, according to calculations, we require a wire cross-section of 2.5 mm 2, look at table 1.3.4 given in the PUE (electrical installation rules). And in our case, let’s say a 6 mm 2 wire was used, the cost of this wire will be 2.5 times higher than 2.5 mm 2, let’s say 2.5 costs 28 rubles, and 6 costs 70 rubles per meter. We will need, say, 20 meters, in the first case we will spend 560 rubles, and in the second 1400 rubles, the difference in money is obvious. Just imagine, if you over-wire the entire apartment, how much money you will throw away. Hence the question, do you need such a reserve?

Summing up the interim results, we learned that incorrect calculation of the wire cross-section has very unpleasant, and in some cases, serious consequences, so it is simply necessary to approach the choice of wire cross-section correctly, competently and seriously.

Formula for calculating wire cross-section

I calculated =P/U nom

where I calculated – calculated current,

P – equipment power,

U nom – rated voltage = 220 volts

For example, let's calculate a 3 kW electric water heater.

3 kW = 3000 W, I calculated =3000/220=13.636363 ..., round I calculation = 14 A

There are also various correction factors depending on the environmental conditions and the laying of the wire, as well as the coefficient of repeated short-term switching on. To a greater extent, these coefficients are important in three-phase networks of 380 volts in production, where large starting currents are present. And in our case, we have household electrical appliances designed for a voltage of 220 volts, so we will not calculate it, but we will definitely take it into account and determine its average value equal to 5 A and add it to the calculated current.

As a result, I calculation = 14 +5 = 19 A,

The wire used is three-core copper (phase, neutral, ground), see the table.

Table of cross-section of copper wires according to long-term permissible current (PUE table 1.3.4)

If the value is in the interval between two currents of different sections, in our case 15 A and 21 A, we always take the larger one. The calculated wire cross-section required to connect a 3 kW water heater is 2.5 mm 2.

So, using the 3 kW water heater shown in the example, we calculated the cross-section of the wires and found out why it is impossible to underestimate and overestimate the cross-section of the wires. We learned how to determine long-term permissible currents, as well as choose the correct wire cross-section.

Similarly, according to the formula, you can also perform this, thanks to which you will achieve optimal illumination without straining your eyesight and high-quality distribution of light flux.

By calculating the wire cross-section with your own hands, you will save:

• When purchasing wires, the cost of the wire increases with the cross-section. For example, 1 meter of a non-flammable wire of a brand that has proven itself quite well in the installation of internal electrical wiring with a cross-section of 1.5 squares costs 15 rubles, and the same wire with a cross-section of 2.5 squares costs 23 rubles, the difference is 8 rubles per meter, from 100 meters this is already 800 rubles.
• On the purchase of protection devices, circuit breakers, RCDs. The higher the operating current of the device, the higher the price. For example, a single-pole circuit breaker for 16 Amps costs 120 rubles, and for 25 Amps it costs 160 rubles, a difference of 40 rubles. The average power panel has about 12 circuit breakers, each costing 40 rubles, the total will be 480 rubles. The difference in the cost of the RCD will be even greater, about 200-300 rubles.